Ln(iii) and ga(iii) metallacrown complexes

ABSTRACT

The present invention concerns heterometallic metallacrown compounds incorporating Ga(III) and Ln(III) cations, with a templating ligand such as salicylhydroxamic acid or derivatives thereof, wherein said metallacrown contains at least one repeating [—Ga—N—O—] sub-unit where the N—O derives from the templating ligand.

The present invention concerns Ln(III) and Ga(III) metallacrown complexes, and uses thereof.

Optical devices, bioanalytical assays, and biological imaging probes often utilize components that exhibit optical properties, such as organic fluorophores and semi-conductor nanoparticles. Some desired optical properties include long luminescence lifetimes, large effective energy differences between excitation and emission bands, and sharp emission bands throughout the visible and near-infrared (NIR) spectral ranges. Lanthanide(III) metal ions (Ln(III) or Ln³⁺) contain 4 f orbitals and exhibit unique luminescent characteristics that fulfill entirely the aforementioned requirements for optical materials. However, most f-f transitions of Ln³⁺ ions are forbidden by quantum mechanics rules inducing low absorption coefficients, resulting in inefficient direct excitation and requiring sensitization with an appropriate antenna.

The antenna sensitization strategy has led to the development of a wide variety of Ln³⁺ based luminescent complexes formed with organic ligands, inorganics complexes and semi-conductors materials. In these complexes, among other parameters, the energy difference (ΔE) between the ligand's excited triplet state (T₁) and the accepting f-orbital electronic level of the lanthanide impacts the global sensitization ability of the antennae.

The aim of the present invention is to provide luminescent lanthanide complexes.

The aim of the invention is to provide lanthanide complexes with remarkable luminescence properties across the visible and near-infrared (NIR) regions.

The present invention relates to metallacrowns incorporating Ga(III) and Ln(III) cations.

For ideal Ln³⁺ based luminescent complexes, it is believed that a good strategy should balance the sensitization efficiency of the Ln³⁺ and the protection from the presence of sources of non-radiative deactivation (due to harmonic combinations of —OH, —NH and —CH vibrations of the solvent molecules and of the organic ligands of the complex). The closer the Ln³⁺ is to the organic chromophoric ligand environment, the larger the efficiency of the sensitization to the Ln³⁺. At the same time, the closer the Ln³⁺ gets to the quenching vibrations of the chromophore, the stronger the non-radiative deactivation. It is believed that the use of rigid systems in the complexes may allow for control of the distance between the antennae and the luminescent lanthanide(III) cations.

The examples disclosed herein are heterometallic metallacrowns which incorporate Ga³⁺ ions.

Metallacrowns are metal complexes made with tetra-dentate ligands that cyclize to form a repeating [-M-N—O—]_(x) sub-unit, where M is a cationic metal that serves as the metallacrown ring metal. Similar to crown ethers, metallacrowns can be synthesized with a range of sizes, and the inward facing oximat oxygen atoms are capable of binding to a central metal ion.

Ga³⁺ ions are an isoelectronic cation to Zn²⁺ that cannot interfere (quench) with the excited states of luminescent lanthanide. It is believed that the Ga³⁺ metallacrowns disclosed herein possess many electronic features similar to Zn²⁺ metallacrowns, while allowing for the use of different chromophores (e.g., tri-anionic metallacrown ligands to compensate for the charge of Ga³⁺).

The Ga³⁺ ring metal forms the backbone of the heterometallic metallacrowns disclosed herein. Various isotopes of gallium have been used for diagnostics and therapeutic biomedical applications. As such, the heterometallic metallacrowns disclosed herein may serve as multipurpose chemical agents. Furthermore, the combination of luminescent Ln³⁺ and radioisotopes of Ga³⁺ allows for the prospect of the heterometallic metallacrowns and complexes being used as both bimodal imaging (the radioisotope ⁶⁸Ga³⁺ is used in PET imaging) and theranostic (other isotopes of Ga³⁺ can be used as potential therapeutic radionuclide) agents.

The present invention relates to a heterometallic metallacrown compound incorporating Ga(III) and Ln(III) cations, said metallacrown containing at least one repeating [—Ga—N—O—] sub-unit.

The present invention relates to a heterometallic metallacrown compound incorporating Ga(III) and Ln(III) cations, with a templating ligand such as salicylhydroxamic acid or derivatives thereof. The said metallacrown contains at least one repeating [—Ga—N—O—] sub-unit where the N—O derives from the templating ligand.

According to one embodiment, the heterometallic metallacrown compound according to the invention includes a Ln(III)[12-MC_(Ga) ^(III) _((N(shi))-4] core, wherein MC_(Ga) ^(III) _(N(shi)) is a metallacrown macrocycle with a repeating sub-unit consisting of Ga(III) ion and a salicylic hydroxamic acid (H₃shi) ligand or its derivatives.

Within the present application, it is to be understood that Ln³⁺ may include any lanthanide ion, such as yttrium (Y³⁺), lanthanum (La³⁺), cerium (Ce³⁺), praseodymium (Pr³⁺), neodymium (Nd³⁺), promethium (Pm³⁺), samarium (Sm³⁺), europium (Eu³⁺), gadolinium (Gd³⁺), terbium (Tb³⁺), dysprosium (Dy³⁺), holmium (Ho³⁺), erbium (Er³⁺), thulium (Tm³⁺), ytterbium (Yb³⁺), or lutetium (Lu³⁺).

According to an embodiment, the heterometallic metallacrown according to the invention may also include at least one counteraction (C⁺) which balances the charge of the compound.

According to an embodiment, the heterometallic metallacrown according to the invention includes at least one ligand as bridging unit between the Ga(III) ion and the Ln(III) ion.

Preferably, said ligand contains at least one carboxylate group COO⁻. Among such ligands, one may cite ligands of formula R—COO⁻, wherein R may be chosen from (C₆-C₁₀)aryl groups or (C₁-C₆)alkyl groups, optionally substituted with an aryl group. In particular, R may be a substituted phenyl, substituted benzyl or substituted methyl group. Preferably, the ligand is a benzoate group.

As other carboxylate bridging units, one may cite isophthalate groups or derivatives thereof.

As derivatives thereof, one may cite groups having the following formula:

with any combination of R-groups bound at each of positions 2, and 4 through 6, each R-group being independently selected from the group consisting of —H, -D, —OH, —SH, —NH₂, —NO₂, —F, —Cl, —Br, —I, —CF₃, —OCH₃, —SO₃H, —CH₃, —CN, a fused aromatic ring, a fused heterocyclic ring, an amide, ═O, ═N, ═N₃, —NR′H, —NR′2, —NR′³⁺, —COOH, —COOR′, —CH₂—R′, —CHR₂, —CHR′R″, —CR′R″R′″, —OR′, and combinations thereof, wherein R′, R″, and R′″ are independently selected from any of the R-groups.

The present invention relates to a heterometallic metallacrown compound, having the formula: Ln(III)(OX)₄[12-MC_(Ga) ^(III) _((N(shi))-4](C⁺), wherein MC_(Ga) ^(III) _(N(shi)) is a metallacrown macrocycle with a repeating sub-unit consisting of Ga(III) ion and a salicylic hydroxamic acid (H₃shi) ligand or its derivatives, OX⁻ are bridging carboxylate units and C+ are countercations that balance the charge of the compound.

According to an embodiment, the above-defined [12-MC_(Ga) ^(III) _(N(shi))-4] unit includes four repeating [Ga(III)shi] sub-units that form a macrocyclic ring having twelve total atoms.

According to an embodiment, the heterometallic metallacrown compound according to the invention includes at least one ligand as bridging unit between the Ga(III) ion and the Ln(III) ion, wherein said ligand is H₂shi. According to this embodiment, the H₂shi acts as a bridge instead of the carboxylate units as mentioned above.

According to an embodiment, the heterometallic metallacrown compounds have the following formula: [Ga₈Ln₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.2H₂O. Such compounds may also be defined by the following formula: Ln₂(isophthalate)₄[12-MC_(GaNshi)-4]₂.

According to an embodiment, the heterometallic metallacrown compounds have the following formula: Ln³⁺ [12-MC_(Ga) ^(III) _(N(shi))-4]. They also may be defined by the following formula: (LnGa₄(shi)₄(H₂shi)₂(C₅H₅N)₄(NO₃)).

According to an embodiment, the heterometallic metallacrown compounds have the following formula: Ga₆Ln(shi)₉ or [LnGa₆(shi)₇(Hshi)(H₂shi)(C₆H₁₆N)₃(C₅H₅N)₂].xH₂O.

According to an embodiment, upon attachment of a targeting moiety or of a group, the heterometallic metallacrown exhibits a shift in its excitation wavelength.

Preferably, the heterometallic metallacrown is configured to emit visible luminescence, near-infrared luminescence, or combinations thereof.

In particular, upon attachment of a targeting moiety or of a group, the heterometallic metallacrown exhibits selective recognition of a cell, cellular substructure, tissue or tumor.

The present invention also relates to the use of the above-mentioned heterometallic metallacrown especially in bioanalytical assays or biological imaging, and also for multi-modal applications.

Within the present application, the templating ligand is salicylhydroxamic acid or derivatives thereof. As derivatives thereof, one may cite the following compounds:

According to an embodiment, the hydroxamic acid ligand and derivatives thereof have the following formula:

with any combination of R-groups bound at each of positions 3 through 6, each R-group being independently selected from the group consisting of —H, -D, —OH, —SH, —NH₂, —NO₂, —F, —Cl, —Br, —I, —CF₃, —OCH₃, —SO₃H, —CH₃, —CN, a fused aromatic ring, a fused heterocyclic ring, an amide, ═O, ═N, —N₃, —NR′H, —NR′2, —NR′³⁺, —COOH, —COOR′, —CH₂—R′, —CHR₂, —CHR′R″, —CR′R″R′″, —OR′, and combinations thereof, wherein R′, R″, and R′″ are independently selected from any of the R-groups.

One example of the heterometallic metallacrown may be referred to herein as an Ga₄Ln(shi)₄ complex (Ln=Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, and Yb³⁺), and has the formula [Ga₄Ln(shi³⁻)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)].(C₅H₆N⁺).C₆H₅N.CH₃OH. The Ln³⁺ is located at the core of the metallacrown and adopts a coordination number of eight. Its coordination is completed through a bond to the ring by benzoate ligands that create a bridge between Ln³⁺ and Ga³⁺ metal ions. In this complex, Ga³⁺ is the ring metal and Ln³⁺ is the central metal. The bridging benzoate ligands may play a role in the sensitization of the lanthanide(III) ions. The Ga₄Ln(shi)₄ complex may also be referred to as Ln^(III)(OBz)₄[12-MC_(Ga) ^(III) _(N(shi))-4](pyr⁺) using nomenclature analogous to crown ethers. The 12 specifies the 12-membered metallacrown ring that has 4 oxime oxygens that may bind to the central cation. MC is used to distinguish a metallacrown from a crown ether (C). The Ga(III) which is the ring metal is specified as a subscript to the MC designator as is N which represents the Nitrogen atom of the oxime which is also an integral component of the metallacrown ring. The ligand that templates the metallacrown is specified as an abbreviation. In this example, shi refers to salicylhydroxamic acid. The central metal is specified outside of the brackets defining the metallacrown to emphasize that the LnIII is captured in the center of the ring. Similarly, bridging anions (in this example benzoate (OBz)) that stabilize the binding of the central metal and also bond to the ring metal are indicated outside the brackets. Counter cations or counteranions not bound to the ring or central metal are designated after the brackets describing the metallacrown. The ring, [12-MC_(Ga) ^(III) _(N(shi))-4], contains twelve atoms, and is a metallacrown macrocycle constituted by four repeating units consisting of Ga³⁺ and the organic ligand salicylic hydroxamic acid (H₃shi) or derivatives thereof. Each of the synthesized Ga₄Ln(shi)₄ heterometallic metallacrowns exhibits lanthanide-based luminescence bands that are specific to the nature of the coordinated lanthanide as a result of an antenna effect located in the metallacrown architecture.

The following scheme is an example of the four-component supramolecular self-assembly process for forming of Ga₄Ln(shi)₄:

In this scheme, at i) salicylic hydroxamic acid, 1 eq. of Ga(NO₃)₃ xH₂O and 0.25 eq. Ln(NO₃)₃ xH₂O are dissolved in MeOH. Other example solvents include dimethylformamide (DMF), water, acetonitrile, and combinations thereof. At ii), 4 eq. of sodium benzoate are added and the solution is stirred. The solution is filtered, and at iii) pyridine is added. The solution is stirred again and filtered. Slow evaporation yields a crystalline compound.

FIGS. 1A and 1B illustrate the X-ray crystal structures of one example of the Ga₄Ln(shi)₄ heterometallic metallacrown where the lanthanide is dysprosium(III) Dy⁺(BzO₂)₄[12-MC_(Ga) ^(III) _(N(shi))-4] (pyr⁺). A top-down along the pseudo C₄-axis is shown in FIG. 1A and a side view perpendicular to the c-axis is shown in FIG. 1B. Hydrogen atoms and lattice solvents have been removed for clarity. The other Ga₄Ln(shi)₄ complexes are identical in composition, except for the lanthanide cation.

Another example of the heterometallic metallacrown may be referred to herein as an Ga₄Ln(shi)₆ complex ([LnGa₄(shi)₄(H₂shi)₂(C₅H₅N)₄(NO₃)](C₅H₅N)₂ (Ln^(III)=Gd^(III), Tb^(III), Dy^(III), and Ho^(III)) and [LnGa₄(shi)₄(H₂shi)₂(C₅H₅N)₅](NO₃)(C₅H₅N) (Ln^(III)=Er^(III), Tm^(III), and Yb^(III))). The Ga₄Ln(shi)₆ complex contains a non-planar Ln³⁺[12-MC_(Ga) ^(III) _(N(shi))-4] core. The central

Ln ion is bridged to two Ga³⁺ ions by two H₂shi⁻ ligands. In the case of Gd^(III), Tb^(III), Dy^(III), and Ho^(III), the lanthanide coordination sphere is further filled by a chelating nitrate.

FIGS. 2A and 2B illustrate the X-ray crystal structure of one example of the [LnGa₄(shi)₄(H₂shi)₂(C₅H₅N)₄(NO₃)](C₅H₅N)₂ heterometallic metallacrowns (i.e., [TbGa₄(shi)₄(H₂shi)₂(C₅H₅N)₄(NO₃)] (C₅H₅N)₂) and its Tb³⁺[12-MC_(Ga) ^(III) _(N(shi))-4] core. Hydrogen atoms and lattice solvents have been removed for clarity. FIGS. 2C and 2D illustrate the X-ray crystal structure of one example of the [LnGa₄(shi)₄(H₂shi)₂(C₅H₅N)₅](NO₃)(C₅H₅N) heterometallic metallacrowns (i.e. [YbGa₄(shi)₄(H₂shi)₂(C₅H₅N)₅](NO₃)(C₅H₅N)) and its Yb³⁺[12-MC_(Ga) ^(III) _(N(shi))-4] core.

Through further modification of reaction conditions (i.e., solvent, stoichiometry, etc.) a third heterometallic metallacrown complex, Ga₆Ln(shi)₉ (Ln=Pr³⁺, Nd³⁺, Sm³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, and Yb³⁺), can be synthesized. The X-ray crystal structure of the Tb³⁺ analog (FIG. 19A) shows a brand new [M-N—O] motif core structure (FIG. 19B). Hydrogen atoms and lattice solvents have been removed for clarity. While metallacrowns are considered to be structural analogs to crown ethers, this complex is analogous to cryptands.

For example, the core [M-N—O] motif of the complex may be compared to 1,10-diaza-2,5,8,12,15,18,20-heptaoxabicylco[8.8.2]icosane (FIG. 19C). Following the cryptate nomenclature, this complex may be described as a Ga(III) [3.3.1] metallacryptand, where Ga2 and Ga5 are considered to be analogous to the nitrogen atoms in a cryptand (FIG. 19C). In combination with metallacrown nomenclature we arrive at the shorthand [Tb(Hshi)(H₂shi){[3.3.1.]MC20-_(Ga) ^(III) _(N(shi))-7}(C₅H₅N)](C₆H₁₆N)₃. The central metal is the terbium ion, the six gallium ions and seven of the shi³⁻ make up the metallacryptate, where there are twenty atoms in the [Ga—N—O] motif, seven of which are oxygens that are distributed across three cryptand-like “arms” in a 3:3:1 ratio. The remaining two shi ligands bridge gallium ions to the terbium, one is singly deprotonated and bridges Ga4 to Tb1 in a “standing up” conformation, the other is doubly deprotonated and bridges Ga3 and Ga6 to the Tb in a “laying down” conformation (FIG. 19A). There is a coordinated pyridine on Ga1. Three triethylammonium cations provide charge balance.

Other heterometallic metallacrowns may be formed with isophthalate bridging compounds.

For example, [Ln₂Ga₈(shi)₈(isophthalate)₄(C₃H₇NO)₆](C₃H₇NO)₈(NH₄ ⁺)₂ complexes (Ln=Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, and Yb³⁺) may be formed. FIG. 4 illustrates the X-ray crystal structure of one example of the [Ln₂Ga₈(shi)₈(isophthalate)₄(C₃H₇NO)₆](C₃H₇NO)₈(NH₄ ⁺)₂ heterometallic metallacrowns (i.e. [Dy₂Ga₈(shi)₈(isophthalate)₄(C₃H₇NO)₆](C₃H₇NO)₈(NH₄ ⁺)₂). These Ln₂Ga₈ complexes can also be formed using derivatives of isophthalate such as 5-maleimidoisophthalate (mip²⁻) and 5-isothiocyanateisophthalate (itip²⁻) (FIG. 26). These functional groups are reactive to couple with amine and thiol-bearing molecules. When employing 5-sulfoisophthalate (sip³⁻), [Na₂Ln₂Ga₈(shi)₈(sip)₄(H₂O)₁₀] (C₃H₇NO)₁₄(C₅H₆N)₂ complexes (Ln=Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, and Yb³⁺) may be formed. FIG. 25 illustrates the X-ray crystal structure of [Na₂Dy₂Ga₈(shi)₈(sip)₄(H₂O)₁₀] (C₃H₇NO)₁₄(C₅H₆N)₂.

As an extension of the examples shown, further modifications to the salicylhydroxamic acid and isophthalic acid ligands may be performed with the intention of tuning the ligand T₁ energy, or allowing for further coupling reactions. As explained above, such modifications include the extension of the aromatic ring of shi to naphthanoic hydroxamic acid, or the replacement of a CH unit with a nitrogen into the aromatic ring as shown in isonicotinic hydroxamic acid. Other modifications on shi and isophthalic acid include the addition of ethynyl, azido, isothiocyano, isocyano, and malimido substituents onto the aromatic rings.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of examples of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear.

FIGS. 1A and 1B are the X-ray crystal structures of Ga₄Dy(shi)₄;

FIGS. 2A and 2B are the X-ray crystal structures of Ga₄Tb(shi)₆ and its Ln³⁺[12-MC_(Ga) ^(III) _(N(shi))-4] core;

FIG. 3 is a graph illustrating the powder X-ray diffraction (PXRD) patterns of Ga₄Ln(shi)₄ complexes, with a simulated pattern obtained from the crystal structure of Ga₄Dy(shi)₄ shown in black;

FIG. 4 is the crystal structure of Ga₈Dy₂(shi)₈ (Ln^(III) ₂(isophthalate)₄[12-MC_(Ga) ^(III) _(N(shi))-4]₂(NH₄ ⁺)₂), where coordinating solvent molecules to the Ga(III) are omitted for clarity;

FIG. 5 is a graph of the UV-Vis absorption spectra for the Ga₄Ln(shi)₄ complexes in methanol at 298 K;

FIG. 6A is a graph of the diffuse reflectance spectra for the solid state Ga₄Ln(shi)₄ complexes presented as Kubelka-Munk function vs. wavelength and FIG. 6B is an enlargement of the spectra in the range 375-780 nm;

FIG. 7 is a graph depicting the absorption spectra of Ga₄Gd(shi)₄, H₃shi and sodium benzoate in methanol at 298 K;

FIG. 8 is a graph illustrating the excitation spectra of the solid state Ga₄Ln(shi)₄ complexes;

FIG. 9 is a graph showing the ligand-based photophysical properties of Ga₄Gd(shi)₄, including the luminescence under excitation at 325 nm at 298 K in methanol solution (black) and solid state (dotted line), and phosphorescence in solid state (grey; λ_(ex)=350 nm, 77 K, 200 μs delay after the excitation flash);

FIG. 10 is a graph depicting emission spectra of solid state Ga₄Ln(shi)₄ complexes under ligand excitation at 350 nm at 298 K;

FIG. 11 is a graph depicting the ligand-based photophysical properties of the solid state Ga₈Gd₂(shi)₈, including the luminescence (black line, λ_(ex)=320 nm, 298 K) and phosphorescence (dotted line; λ_(ex)=350 nm, 77 K, 100 μs delay after the excitation flash);

FIG. 12 is a graph depicting the excitation spectra of the solid state Ga₈Ln₂(shi)₈ complexes at 298K;

FIG. 13 is a graph depicting emission spectra of the solid state Ga₈Ln₂(shi)₈ under ligand excitation at 350 nm at 298 K;

FIGS. 14A and B are microscope images of Ga₈Yb₂(shi)₈ when incubated with HeLa cells for 3 hours at 10 μM concentration, where FIG. 14A represents brightfield (left), NIR emission (center; λ_(ex): 377 nm bandpass 50 nm filter, λ_(em): long pass 805 nm filter, exposure time: 25 s) and merged (right) images, and FIG. 14B represents a control where no MC was added upon incubation;

FIG. 15 is a graph depicting the UV/Vis absorption spectra of H₃shi and Ga₄Ln(shi)₆ molecules in methanol at 298 K;

FIG. 16A is a graph depicting the diffuse reflectance spectra of solid state Ga₄Ln(shi)₆ complexes presented as Kubelka-Munk function vs. wavelength and FIG. 16B is an enlargement of the spectra in the range 375-780 nm;

FIG. 17 is a graph depicting the excitation spectra of Ga₄Ln(shi)₆ complexes upon monitoring main emission transition of the corresponding lanthanide(III) ions;

FIG. 18 is a graph depicting the emission spectra of Ga₄Ln(shi)₆ complexes upon ligand excitation;

FIG. 19A is the crystal structure of Ga₆Dy(shi)₉;

FIGS. 19B and 19C are representations of the two gallium 12-MC-4s of Ga₆Dy(shi)₉; one involves Ga1, Ga2, Ga3, and Ga6 (FIG. 19B); the other involves Ga1, Ga3, Ga4 and Ga5 (FIG. 19C);

FIG. 20 is a graph depicting the UV/Vis absorption spectra of Ga₆Ln(shi)₉ (Ln=Tb, Sm, or Dy) molecules at 298 K;

FIG. 21A is a graph depicting the diffuse reflectance spectra of Gd₄Ln(shi)₉ presented as Kubelka-Munk function vs. wavelength and FIG. 21B is an enlargement of the spectra in the range 375-780 nm;

FIG. 22 is a graph depicting the excitation spectra of Ga₄Ln(shi)₉ complexes upon monitoring main emission transition of the corresponding lanthanide(III) ions;

FIG. 23 is a graph depicting the emission spectra of Ga₄Ln(shi)₉ complexes upon ligand excitation;

FIG. 24 is the crystal structure of Ga₄Yb(shi)₆ with no bound nitrate;

FIG. 25 is the crystal structure of Ga₈Dy₂(shi)₈ with the SO₃ groups on isophthalates;

FIG. 26 is the crystal structure of possible modifications of Ga₈Ln₂(shi)₈ structure on bridging isophthalate ligands;

FIG. 27 concerns the mass-spectrum of [Yb₂Ga₈(shi)₈(mip)₄](CH₃OH)₈(C₅H₆N⁺)₂;

FIG. 28 concerns the mass-spectrum of [Yb₂Ga₈(shi)₈(itip)₄](CH₃OH)₈(C₅H₆N⁺)₂;

FIG. 29 concerns the mass-spectrum of [Yb₂Ga₈(shi)₈(L-cysteine-mip)₄](CH₃OH)₈(C₅H₆N⁺)₂;

FIG. 30 concerns the mass-spectrum of Yb₂Ga₈(shi)₈(cysteamine-mip)₄](CH₃OH)₈(C₅H₆N⁺)₂;

FIG. 31 is the ¹H-NMR of the biotin-eshi coupled molecule (example 1);

FIG. 32 is the ESI-MS+ of the biotin-eshi coupled molecule (example 1): parent peak present at 504.2; sodium adduct at 526.2.

To further illustrate the present disclosure, examples are given herein. It is to be understood that these examples are provided for illustrative purposes and are to be construed as non-limiting.

EXAMPLES Example 1

All reagents and chemicals were purchased from commercial sources and used without further purification. CHN analysis was performed by Atlantic Microlabs Inc. All reactions were carried under aerobic conditions.

Preparation of Ga₄Ln(shi)₄ Complexes

H₃shi (153.1 mg, 1.0 mmol), Ln(NO₃)₃.xH₂O (0.25 mmol; 0.50 mmol for Ln=Sm and Eu), and Ga(NO₃)₃.xH₂O (225.7 mg, 1.0 mmol) were dissolved in 40 mL methanol. Sodium benzoate (576.4 mg, 4.0 mmol) was added to the solution and stirred overnight. The solution was filtered, followed by addition of 2 mL pyridine. The solution was stirred for 15 minutes and then filtered. Slow evaporation of the half of the solution yielded crystalline compound after 2 weeks.

[Ga₄Sm(shi)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)].C₅H₆N.C₅H₅N.CH₃OH (Ga₄Sm(shi)₄). Yield: 104.9 mg (20.1%). ESI-MS, calc. for [M]⁻, C₅₆H₃₆N₄O₂₀GdGa₄, 1519.8; found, 1519.8. Anal. Calcd for SmGa₄C₇₃H₆₀N₇O₂₂: C, 48.27; H, 3.33; N, 5.40. Found: C, 48.29; H, 3.16; N, 5.51.

[Ga₄Eu(shi)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)].C₅H₆N.C₅H₅N.CH₃OH (Ga₄Eu(shi)₄). Yield: 213.0 mg (20.1%). ESI-MS, calc. for [M]⁻, C₅₆H₃₆N₄O₂₀GdGa₄, 1519.8; found, 1519.8. Anal. Calcd for EuGa₄C₇₃H₆₀N₇O₂₂: C, 48.22; H, 3.33; N, 5.39. Found: C, 48.35; H, 3.13; N, 5.58.

[Ga₄Gd(shi)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)].C₅H₆N.C₅H₅N.CH₃OH (Ga₄Gd(shi)₄). Yield: 91.8 mg (20.1%). ESI-MS, calc. for [M]⁻, C₅₆H₃₆N₄O₂₀GdGa₄, 1519.8; found, 1519.8. Anal. Calcd for GdGa₄C₇₃H₆₀N₇O₂₂: C, 48.09; H, 3.32; N, 5.38. Found: C, 48.18; H, 3.07; N, 5.57.

[Ga₄Tb(shi)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)].C₅H₆N.C₅H₅N.CH₃OH (Ga₄Tb(shi)₄). Yield: 102.0 mg (22.4%). ESI-MS, calc. for [M]⁻, C₅₆H₃₆N₄O₂₀TbGa₄, 1522.8; found, 1522.8. Anal. Calcd for TbGa₄C₇₃H₆₀N₇O₂₂: C, 48.04; H, 3.31; N, 5.37. Found: C, 48.33; H, 3.12; N, 5.54.

[Ga₄Dy(shi)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)].C₅H₆N.C₅H₅N.CH₃OH (Ga₄Dy(shi)₄). Yield: 106.6 mg (23.3%). ESI-MS, calc. for [M]⁻, C₅₆H₃₆N₄O₂₀DyGa₄, 1525.8; found, 1525.8. Anal. Calcd for DyGa₄C₇₃H₆₀N₇O₂₂: C, 47.95; H, 3.31; N, 5.36. Found: C, 48.08; H, 3.10; N, 5.54.

[HoGa₄(shi)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)].C₅H₆N.C₅H₅N.CH₃OH (Ga₄Ho(shi)₄). Yield: 160.4 mg (35.0%). ESI-MS, calc. for [M]⁻, C₅₆H₃₆N₄O₂₀HoGa₄, 1528.8; found, 1529.3. Anal. Calcd for HoGa₄C₇₃H₆₀N₇O₂₂: C, 47.88; H, 3.30; N, 5.35. Found: C, 48.01; H, 3.07; N, 5.50.

[ErGa₄(shi)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)].C₅H₆N.0.5C₅H₅N (Ga₄Er(shi)₄). Yield: 159.5 mg (36.2%). ESI-MS, calc. for [M]⁻, C₅₆H₃₆N₄O₂₀ErGa₄, 1529.8; found, 1530.1. Anal. Calcd for ErGa₄C_(69.5)H_(53.5)N_(6.5)O₂₁: C, 47.38; H, 3.06; N, 5.17. Found: C, 47.29; H, 3.05; N, 5.53.

[TmGa₄(shi)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)].C₅H₆N.0.5C₅H₅N (Ga₄Tm(shi)₄). Yield: 148.4 mg (33.6%). ESI-MS, calc. for [M]⁻, C₅₆H₃₆N₄O₂₀TmGa₄, 1532.8; found, 1532.8. Anal. Calcd for TmGa₄C_(69.5)H_(53.5)N_(6.5)O₂₁: C, 47.33; H, 3.06; N, 5.16. Found: C, 47.06; H, 2.95; N, 5.48.

[YbGa₄(shi)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)].C₅H₆N.C₅H₅N.CH₃OH (Ga₄Yb(shi)₄). Yield: 54.1 mg (11.8%). ESI-MS, calc. for [M]⁻, C₅₆H₃₆N₄O₂₀YbGa₄, 1535.8; found, 1535.8. Anal. Calcd for YbGa₄C₇₃H₆₀N₇O₂₂: C, 47.67; H, 3.29; N, 5.33. Found: C, 47.69; H, 3.10; N, 5.49.

Preparation of [Ln₂Ga₈(shi)₈(isophthalate)₄(C₃H₇NO)₆](C₃H₇NO)₈(NH₄ ⁺)₂ Complexes

H₃shi (306.3 mg, 2.0 mmol), Ln(NO₃)₃.xH₂O (0.50 mmol), Ga(NO₃)₃.xH₂O (511.5 mg, 2.0 mmol), and isophthalic acid (166.1 mg, 1.0 mmol) were dissolved in 15 mL DMF. Ammonium bicarbonate (632.5 mg, 8.0 mmol) was added to the solution and stirred overnight. The solution was filtered. Slow evaporation of the half of the solution yielded crystalline compound after 3 months.

[Ga₈Pr₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.2H₂O (Ga₈Pr₂(shi)₈). Yield: 33.9 mg (3.6%). Anal. Calcd for Pr₂Ga₈C₁₃₀H₁₅₈N₂₄O₅₆: C, 41.17; H, 4.20; N, 8.86. Found: C, 40.98; H: 4.46; N: 9.22. Unit Cell: a=14.193 Å, b=17.696 Å, c=19.169 Å; α=113.22°, β=103.24°, γ=97.65°; V=4170.37 Å³.

[Ga₈Nd₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.H₂O (Ga₈Nd₂(shi)₈). Yield: 98.0 mg (10.4%). Anal. Calcd for Nd₂Ga₈C₁₃₀H₁₅₆N₂₄O₅₅: C, 41.30; H, 4.16; N, 8.89. Found: C, 40.96; H: 4.39; N: 9.36. Unit Cell: a=14.218 Å, b=17.725 Å, c=19.251 Å; α=113.19°, β=103.33°, γ=97.63°; V=4201.57 Å³.

[Ga₈Sm₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.2H₂O (Ga₈Sm₂(shi)₈). Yield: 195.6 mg (20.5%). Anal. Calcd for Sm₂Ga₈C₁₃₀H₁₅₈N₂₄O₅₆: C, 40.97; H, 4.18; N, 8.82. Found: C, 40.67; H: 4.51; N: 8.96. Unit Cell: a=14.152 Å, b=17.741 Å, c=19.188 Å; α=112.91°, β=102.71°, γ=98.27°; V=4185.38 Å³.

[Ga₈Gd₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.H₂O (Ga₈Gd₂(shi)₈). Yield: 321.0 mg (33.7%). Anal. Calcd for Gd₂Ga₈C130H₁₅₆N₂₄O₅₅: C, 41.01; H, 4.13; N, 8.83. Found: C, 40.98; H: 4.28; N: 8.99. Unit Cell: a=14.104 Å, b=17.581 Å, c=19.217 Å; α=113.09°, β=102.60°, γ=98.35°; V=4134.44 Å³.

[Ga₈Tb₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.H₂O (Ga₈Tb₂(shi)₈). Yield: 329.8 mg (34.6%). Anal. Calcd for Tb₂Ga₈C₁₃₀H₁₅₆N₂₄O₅₅: C, 40.98; H, 4.13; N, 8.82. Found: C, 40.62; H: 4.45; N: 8.81. Unit Cell: a=14.105 Å, b=17.595 Å, c=19.248 Å; α=113.24°, β=102.63°, γ=98.16°; V=4142.24 Å³.

[Ga₈Dy₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.2H₂O (Ga₈Dy₂(shi)₈). Yield: 280.1 mg (34.6%). Anal. Calcd for Dy₂Ga₈C₁₃₀H₁₅₈N₂₄O₅₆: C, 40.71; H, 4.15; N, 8.76. Found: C, 40.75; H: 4.45; N: 8.87. Unit Cell: a=14.1080 Å, b=17.5806 Å, c=19.2197 Å; α=113.107°, β=102.699°, γ=98.218°; V=4135.39 Å³.

[Ga₈Ho₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.2H₂O (Ga₈Ho₂(shi)₈). Yield: 322.0 mg (33.7%). Anal. Calcd for Ho₂Ga₈C₁₃₀H₁₅₈N₂₄O₅₆: C, 40.66; H, 4.15; N, 8.75. Found: C, 40.31; H: 4.38; N: 9.02. Unit Cell: a=14.114 Å, b=17.658 Å, c=19.239 Å; α=113.06°, β=102.65°, γ=98.26°; V=4161.85 Å³.

[Ga₈Er₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.2H₂O (Ga₈Er₂(shi)₈). Yield: 276.2 mg (28.7 %). Anal. Calcd for Er₂Ga₈C₁₃₀H₁₅₈N₂₄O₅₆: C, 40.61; H, 4.14; N, 8.74. Found: C, 40.36; H: 4.42; N: 8.98. Unit Cell: a=14.092 Å, b=17.504 Å, c=19.208 Å; α=113.16°, β=102.51°, γ=98.25°; V=4112.80 Å³.

[Ga₈Tm₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.2H₂O (Ga₈Tm₂(shi)₈). Yield: 235.0 mg (24.4%). Anal. Calcd for Tm₂Ga₈C₁₃₀H₁₅₈N₂₄O₅₆: C, 40.57; H, 4.14; N, 8.73. Found: C, 40.18; H: 4.26; N: 8.98. Unit Cell: a=14.148 Å, b=17.718 Å, c=19.296 521 ; α=113.08°, β=102.58°, γ=98.30°; V=4198.67 Å³.

[Ga₈Yb₂(shi)₈(isophthalate)₄(DMF)₆].8DMF.3H₂O (Ga₈Yb₂(shi)₈). Yield: 200.5 mg (20.7%). Anal. Calcd for Yb₂Ga₈C₁₃₀H₁₆₀N₂₄O₅₇: C, 40.30; H, 4.16; N, 8.68. Found: C, 39.92; H: 4.27; N: 8.88. Unit Cell: a=14.110 Å, b=17.700 Å, c=19.242 Å; α=113.05°, β=102.48°, γ=98.35°; V=4173.97 Å³.

Physical Methods

X-Ray Crystallography

Crystal data for the compounds Ga₄Dy(shi)₄, Ga₄Tb(shi)₆ (discussed further in example 2), and Ga₆Tb(shi)₉ were collected at 85(2) K on an AFC10K Saturn 944+ CCD-based X-ray diffractometer equipped with a Micromax007HF Cu-target microfocus rotating anode (λ=1.54187 Å), operated at 1200 W power (40 kV, 30 mA). The data were processed with CrystalClear 2.0 and corrected for absorption. The structure was solved and refined with the SHELXTL (version 6.12) software package. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms placed in their idealized positions. For Ga₄Tb(shi)₆, the structure contained large solvent accessible voids totaling 834.1 Å³ and 247.5 electrons per unit cell. This region had diffuse electron density and could not be modelled with any chemically reasonable moieties. The SQUEEZE routine of the PLATON suite of programs was applied to remove the diffraction contribution from these solvents.

Powder X-ray Diffraction (PXRD)

Powder X-ray diffraction data for air-dried samples of the Ga₄Ln(shi)₄ complexes were collected at room temperature using a Bruker D8 Advance Diffractometer with Cu—K_(α) radiation (1.5406 Å, 40 kV, 40 mA). Powder diffraction patterns were collected at room temperature from 3° to 50° (2θ) using a step size of 0.05° and a scan time of 0.5 second/step.

Solid State Diffuse Reflectance

Solid state UV-Vis spectra were collected using an Agilent-Cary 5000 spectrophotometer equipped with a Praying Mantis diffuse reflectance accessory. Spectra were collected in reflectance mode, with BaSO₄ was used as a baseline. Samples (10% by weight) were mulled in BaSO₄ (90% by weight). The spectra were converted into normalized absorbance by using the equation A=1−R.

Solution Absorption Spectra

UV-Vis spectra for the compounds dissolved in methanol were recorded on a Cary 100Bio UV-Vis spectrophotometer. All spectra were collected in absorbance mode.

Photophysical Measurements

Luminescence data were collected on samples placed into 2.4 mm i.d. quartz capillaries or quartz Suprasil cells. Emission and excitation spectra were measured on a Horiba-Jobin-YvonFluorolog 3 spectrofluorimeter equipped with either a visible photomultiplier tube (PMT) (220-800 nm, R928P; Hamamatsu), a NIR solid-state InGaAs detector cooled to 77 K (800-1600 nm, DSS-IGA020L; Jobin-Yvon), or NIR PMTs (950-1450 nm, H10330-45; 950-1650 nm, H10330-75; Hamamatsu). All spectra were corrected for instrumental functions. Luminescence lifetimes were determined under excitation at 355 nm provided by a Nd:YAG laser (YG 980; Quantel), while the signal was detected in the NIR by the aforementioned PMT (H10330-75). The output signal from the detectors was then fed to a 500 MHz bandpass digital oscilloscope (TDS 754C; Tektronix) and then transferred to a PC for treatment with Origin 8®. Luminescence lifetimes are averages of at least three independent measurements. Quantum yields were determined with a Fluorolog 3 spectrofluorimeter according to an absolute method using an integration sphere (GMP SA). Each sample was measured several times under slightly different experimental conditions. Estimated experimental error for quantum yields determination is 10%.

Synthesis and Characterization

Ga₄Ln(shi)₄ Complexes

The reaction between shi, gallium(III) nitrate, lanthanide(III) nitrate, and sodium benzoate in a mixed methanol/pyridine solvent resulted in the formation of a metallacrown that adopts the 12-MC-4 topology, the negative charge of which is counter-balanced by a pyridinium cation. The success of this synthesis depends strongly on the size of the Ln³⁺, with Gd³⁺ being the largest ion that be can be incorporated. Larger ions may not fit within the cavity of the metallacrown resulting in lower thermodynamic stability. The X-ray crystal structure for the Ga₄Dy(shi)₄ compound is shown in FIGS. 1A and 1B. The X-ray crystal structure of Ga₄Dy(shi)₄ was determined to be in P2₁/n.

The other Ga₄Ln(shi)₄ complexes were determined to be identical in composition by elemental analysis, ESI-MS, and PXRD. For each metallacrown, four shi³⁻ ligands coordinate four Ga³⁺ ions to form the 12-MC-4 ring.

The central Dy³⁺ ion is bridged to the ring by four benzoate ligands and adopts an 8-coordinate geometry which is close to a square antiprism. The pyridinium counter-cation was determined to be protonated to adjacent solvent molecules through hydrogen bonding.

In the previously reported LnZn₁₆L₁₆ structures, the distance between the Ln³⁺ ion and the closest C—H oscillator was larger than 6 Å. The presence of high energy C—H, N—H, and O—H oscillators in close proximity to the Ln³⁺ can lead to the quenching of the NIR luminescence. For Ga₄Dy(shi)₄, and the other analogues of the series, the shortest Ln-CH distance was found to be 4.37 Å. Nevertheless, remarkable NIR photophysical properties have been observed for these complexes despite this relatively short distance. This is a qualitative indication that the effect of potential luminescence quenching is compensated by the efficient chromophore to lanthanide energy transfer.

The centroid in the space between the four benzoate groups is ˜2.3 Å from the nearest (i.e., closest) hydrogen atoms. Although no electron density was observed in that region, this void space is large enough to be occupied by a solvent molecule, which would potentially have implications in the photophysical data for the Ga₄Ln(shi)₄ complexes.

The other Ln³⁺ analogues were confirmed to be isostructural by powder X-ray diffraction (FIG. 3).

Ga₈Ln₂(shi)₈ Complexes

Utilizing a synthetic strategy, 12-MC-4 unit in the Ga₄Ln(shi)₄ complexes were linked together via isophthalate ligands. Using ammonium bicarbonate as a base, reaction of H₃shi, Ln(NO₃)₃.xH₂O, Ga(NO₃)₃.xH₂O and isophthalic acid in DMF forms the Ga₈Ln₂ complex which can be described as two 12-MC-4 monomer subunits connected via four isophthalate groups (see FIG. 4). The molecular moiety has a net dianionic charge. Since no other metal atoms could be found in the electron density map, the charge is likely balanced by two lattice NH₄ ⁺ ions. However, these counter-ions could not be located due to weak scattering. Similarly, the diffuse electron density of the lattice solvent required the use of SQUEEZE routine of the PLATON suite of programs.

TABLE 1 Crystallographic details for Ga₈Dy₂(shi)₈ Ga₈Dy₂ mol formula C₉₆H₂₀Dy₂Ga₈N₁₂O₄₆ Mw (g/mol) 2939.84   Triclinic/P-1 T (K) 85(2)   wavelength (Å)  1.54178 a (Å) 14.1080(3) b (Å) 17.5806(3) c (Å)  19.2197(14) α (deg) 113.107(8)  β (deg) 102.699(7)  γ (deg) 98.218(7)  V (Å³) 4135.4(3)    Z 1   density, ρ (g/cm³) 1.180 abs coeff, μ (mm⁻¹) 6.713 F(000) 1408     θ range for data collection (deg) 2.62 to 68.24 limiting indices −16 ≤ h ≤ 16 −20 ≤ k ≤ 21 −23 ≤ l ≤ 23 reflns collected/unique 111713/14938 completeness to θ (%) 98.6   no. of data/restraints/parameters 14938/0/815 goodness of fit on F² 1.929 final R indices R₁ ^(a) = 0.1430 [I > 2σ(I)] wR₂ ^(b) = 0.3897 R indices (all data) R₁ ^(a) = 0.1459 wR₂ ^(b) = 0.3981 largest diff peak and hole (e⁻ Å⁻³) 12.150 and −2.064 ^(a)R₁ = Σ(||F_(o)| − |F_(c)||)/Σ|F_(o)|. ^(b)wR₂ = [Σ[w(F_(o) ² − F_(c) ²)²]/Σ[w(F°)²]]^(1/2); w = 1/[σ²(F_(o) ²) + (mp)² + np]; p = [max(F_(o) ², 0) + 2F_(c) ²]/3 (m and n are constants); σ = [Σ[w(F_(o) ² − F_(c) ²)²]/(n − p)]^(1/2)

TABLE 2 Selected bond lengths in the crystal structure of Ga₈Dy₂ Bond Length (Å) Dy(1)—O(5) 2.301(10) Dy(1)—O(2) 2.326(8) Dy(1)—O(112) 2.328(9) Dy(1)—O(111) 2.343(8) Dy(1)—O(101) 2.347(7) Dy(1)—O(103) 2.353(8) Dy(1)—O(8) 2.391(9) Dy(1)—O(11) 2.396(7)

Since the Ga₈Dy₂ complex crystallizes in the space group P-1, the two Dy³⁺ ions are symmetry related by an inversion center and are situated 7.23 Å apart, such that any through-space interactions are likely negligible. As with Ga₄Ln(shi)₄, the closest C—H oscillator resides on the nearest carbon of a bridging benzoate and is 4.51 Å apart from the nearest Dy³⁺ ion. This distance is slightly longer than the 4.37 Å observed for Ga₄Dy(shi)₄.

Photophysical Properties

Ga₄Ln(shi)₄ Complexes

All synthesized complexes (excluding Ga₄Gd(shi)₄) of the Ga₄Ln(shi)₄ series showed the ability to emit characteristic visible (Ga₄Tm(shi)₄, Ga₄Dy(shi)₄, Ga₄Tb(shi)₄, Ga₄Sm(shi)₄, Ga₄Eu(shi)₄) and/or near-infrared luminescence (Ga₄Dy(shi)₄, Ga₄Yb(shi)₄, Ga₄Ho(shi)₄, Ga₄Er(shi)₄, Ga₄Sm(shi)₄). For each of these metallacrowns, the bands observed in both the solution absorption (FIG. 5) and solid state diffuse reflectance (FIG. 6) spectra match well with those observed in the excitation spectra of the different Ga₄Ln(shi)₄ complexes (FIG. 8). This indicates that the excitation light is absorbed by the chromophoric H₃shi ligands and that the corresponding energy is being transferred to the luminescent lanthanides. Thus, this metallacrown design is able to provide an antenna effect for these different lanthanide cations emitting in the visible and in the NIR.

The Gd³⁺ derivative in this series is a useful probe to assess the ligand-centered photophysical properties of these metallacrowns, since this cation is not expected to exhibit Ln³⁺ luminescence as the energy of the triplet state of the shi³⁻ ligand is hypothesized to be too low to transfer to the accepting Gd³⁺ level. The absorption spectrum of Ga₄Gd exhibits two major features attributed to the ligand-based π→π* transitions with the lower energy absorption band located at 310 nm (corresponding to an energy of ca. 32,250 cm⁻¹, FIG. 7). This band is red-shifted by 11 nm compared to the one of the ligand H₃shi, which exhibits low-energy π→π* absorption centered at 299 nm (FIG. 7); sodium benzoate does not show absorption bands below ca. 285 nm (FIG. 7).

All of the synthesized Ga₄Ln(shi)₄ complexes exhibit similar solution absorption spectra indicating that the nature of the lanthanide ion does not affect the electronic properties of the resulting complex (FIG. 5). In diffuse reflectance spectra of solid state Ga₄Ln(shi)₄ complexes (FIG. 6) apart from broad-band ligand-centered transitions sharp characteristic bands which can be attributed to the corresponding f-f transitions of Sm³⁺, HO³+, Er³⁺ and Tm³⁺ are observed. Moreover, in the case of Ga₄Eu(shi)₄, ligand-to-metal charge transfer (LMCT) can be detected as an extension of the band to longer wavelengths (FIG. 6). The LMCT cannot be observed in the solution UV-Vis spectra due to its low molar absorption coefficient.

Under excitation at 325 nm in solution (CH₃OH) at room temperature, Ga₄Gd(shi)₄ exhibits fluorescence arising from its chromophoric moieties at 367 nm (ca. 27,250 cm⁻¹, FIG. 9), which is 5000 cm⁻¹ lower in energy than the absorption band. Phosphorescence can be observed by recording the signal emitted from Ga₄Gd(shi)₄ in time-resolved mode in the solid state at 77K upon excitation at 350 nm and using a 200 μS delay after the excitation flash (FIG. 9). The 0-0 component of this band represents the energy level of the ligand's triplet state (T₁=22,170 cm⁻¹, 451 nm) which is attributed to the energy level responsible for the main contribution of the transfer to the Ln³⁺. The Ti level is sufficiently high in energy to populate the excited states of a wide range of visible and NIR-emitting Ln³⁺. However, the energy difference (αE) between the donating triplet state of the reported metallacrowns and the main accepting emitting levels of Tm³⁺ (¹G₄, 21 350 cm⁻¹), Tb³⁺ (⁵D₄, 20 400 cm⁻¹) and Dy³⁺ (⁴F_(9/2), 21 100 cm⁻¹) is relatively small (<2 000 cm⁻¹), suggesting that back energy transfer processes from the luminescent lanthanide to the chromophoric ligand is possible as discussed below.

Emission spectra of Ga₄Tm(shi)₄, Ga₄Sm(shi)₄, Ga₄Eu(shi)₄, Ga₄Dy(shi)₄, Ga₄Tb(shi)₄, Ga₄Ho(shi)₄, Ga₄Yb(shi)₄, and Ga₄Er(shi)₄ were collected in solid state samples (FIG. 10), and in MeOH or MeOD solutions when possible (not shown). The results of this photophysical study are summarized in Table 3 below. Excitation spectra recorded on solid samples (FIG. 8) and on solutions (not shown) are dominated by broad-bands up to 380-400 nm due to the ligand-based transitions. In addition, sharp bands which can be assigned to f-f transitions were observed for the Ga₄Tm(shi)₄, Ga₄Sm(shi)₄, Ga₄Eu(shi)₄, Ga₄Dy(shi)₄, Ga₄Tb(shi)₄, Ga₄Ho(shi)₄, and Ga₄Er(shi)₄ samples recorded in the solid state, though they are less pronounced for the Ga₄Tb(shi)₄ with a lower relative intensity. These f-f transitions were not observed in solution.

One parameter that can be obtained from the recording of luminescence lifetimes is the hydration number q. Estimations have been developed by comparing lifetimes in deuterated and protic solvents, and are summarized in Table 3. The hydration number was calculated to be between ca. 0.7 and 1.2, indicating that the Ln³⁺ is coordinated to one molecule of solvent. This non-zero q value is particularly detrimental for the intensity of the luminescence (and to the corresponding quantum yield values) of NIR-emitting lanthanides since the overtones of high-energy vibrations of the solvent molecules create a route for non-radiative deactivation. These results are in accordance with the previous discussion of the crystal structure of Ga₄Ln: it is possible that the void space between bridging benzoate molecules may be occupied by a solvent molecule.

TABLE 3 Photophysical data for Ga₄Ln(shi)₄ MC complexes. Q_(Ln) ^(L) (%) Q_(Ln) ^(L) (%) MC State/Solvent ΔE (cm⁻¹)^(a) τ_(obs) (μs) q (visible) (NIR) Ga₄Sm(shi)₄ Solid 4 270 148(1)   0.8^(b) 2.46(8) 0.450(4) CD₃OD 255(1)   2.33(5) 0.298(1) CH₃OH 27.6(1)  0.252(1) 2.65(6) · 10⁻² Ga₄Eu(shi)₄ Solid 4 910 242(7): 79% 1.59(4) · 10⁻² —  43(2): 21% Ga₄Tb(shi)₄ Solid 1 770 1080(10)   1.2^(c) 34.7(1)  — CD₃OD 1960(10)   0.9^(d) 28.6(1)  — CH₃OH 1510(10)   23.7(3)  — Ga₄Dy(shi)₄ Solid 1 070 21.2(2) 0.7^(e) 2.10(1) 0.21(1) CD₃OD 25.5(7) 0.78(1) 6.0(1) · 10⁻² CH₃OH 12.0(1) 0.38(1) 2.4(1) · 10⁻² Ga₄Ho(shi)₄ Solid 6 670   0.029(1) — 2.0(2) · 10⁻³ Ga₄Er(shi)₄ Solid 15 470   6.75(3) — 4.4(1) · 10⁻² CD₃OD  1.74(1) — 4.5(3) · 10⁻² Ga₄Tm(shi)₄ Solid  820  1.47(1)  0.02(1)^(f) Ga₄Yb(shi)₄ Solid 11 870  55.7(3) — 5.88(2) CD₃OD 36.6(1) 0.7^(g) — 4.29(1) CH₃OH  2.06(4) — 0.26(1) ^(a)ΔE(T − E^(Ln)) is the energy gap between the ligand triplet state (T₁ = 22 170 cm⁻¹) and Ln³⁺ emissive state: E^(Sm) (⁴G_(5/2)) = 17 900 cm⁻¹, E^(Eu) (⁵D₀) = 17 260 cm⁻¹, E^(Tb) (⁵D₄) = 20 400 cm⁻¹, E^(Dy) (⁴F_(9/2)) = 21 100 cm⁻¹, E^(Ho) (⁵F₅) = 15 500 cm⁻¹, E^(Er) (⁴I_(13/2)) = 6 700 cm⁻¹, E^(Tm) (¹G₄) = 21 350 cm⁻¹, E^(Yb) (²F_(5/2)) = 10 300 cm⁻¹. ^(b)q_(Sm) = 2 × [25.4 × (k_(CH) ₃ _(OH) − k_(CD) ₃ _(OD)) − 0.37] in μs. ^(c)q_(Tb) = 8.2 × (k_(CH) ₃ _(OH) − k_(CD) ₃ _(OD)) in ms. ^(d)q_(Tb) = 10.0 × (k_(CH) ₃ _(OH) − k_(CD) ₃ _(OD) − 0.06) in ms. ^(e)q_(Dy) = 2 × [21.1 × (k_(CH) ₃ _(OH) − k_(CD) ₃ _(OD)) − 0.6] in μs. ^(f)Quantum yield of Tm-centered transitions after the subtraction of the ligand-based signal; the total quantum yield of Ga₄Tm(shi)₄ is 0.12(1)%. ^(g)q_(Yb) = 2 × (k_(CH) ₃ _(OH) − k_(CD) ₃ _(OD) − 0.1) in μs.

The Ga₄Eu(shi)₄ complex exhibited transitions between the ⁵D₀ emitting state to ⁷F₀₋₄ states between 575 nm and 725 nm (FIG. 10) in the solid state, which are typical for Eu³⁺ compounds. The relatively small quantum yield and short luminescent lifetime observed (Table 3) are likely due to the quenching effect of LMCT states.

The main emitting state for Sm³⁺ ion is ⁴G_(5/2), which is only slightly higher in energy than the ⁵D₀ state for Eu³⁺. In both the solid state and in solution characteristic Sm³⁺ emission arising from ⁴G_(5/2)→⁶H_(J) (J=5/2, 7/2, 9/2 and 11/2) transitions in the visible between 550 nm and 750 nm and ⁴G_(5/2)→⁶F_(J) (J=1/2−11/2) transitions in the NIR (>800 nm) ranges in clearly observed (FIG. 10). The quantum yields for the visible emission in solution (2.33% in CD₃OD) and in the solid state (2.46%) are modest; as quantum yields up to 11% have been reported. However, the lifetime in CD₃OD (255 μs) is quite long. Furthermore, the NIR emission corresponding to ⁴G_(5/2)→⁶F_(J) (J=1/2−11/2) transitions (FIG. 10) are intense enough to obtain quantum yields in both the solid state and in solution (Table 3), which is rare.

The observation of an emission signal arising from Tm³⁺ in molecular complexes that are formed with organic ligands is highly rare. The emission spectrum obtained for Ga₄Tm(shi)₄ (FIG. 10) recorded on a solid state sample shows two visible characteristic bands originating from the ¹G₄ energy level and terminating on the ³H₆ and ³F₄ ground state levels. The signal of the former transition overlaps with a broad organic emission arising from the chromophoric ligand as the result of a not complete ligand to lanthanide energy transfer or back transfer. The NIR bands of Tm³⁺ could not be observed. Furthermore, no characteristic Tm³⁺ visible or NIR emission was observed from this sample in solution, which could be explained by an increased role of quenching processes.

The emission spectrum of Ga₄Dy(shi)₄ recorded on solids-state samples (FIG. 10) exhibits a number of sharp bands across the visible and NIR regions, originating from electronic transitions between the excited ⁴F_(9/2) energy level and the ⁶H_(J) (J=15/2−5/2) and ⁶F_(J) (J=7/2−1/2) ground state levels. In solution, a residual emission signal from the organic ligands was also observed, indicating an incomplete energy transfer to Dy³⁺ or more efficient back energy transfer. Quantum yields recorded for Ga₄Dy(shi)₄ in the visible and the NIR are reported in Table 3. This result is believed to be the first quantitative report of NIR emission arising from Dy³⁺.

Relatively strong emission bands resulting from transitions between the ⁵D₄ level and terminating at the ⁷F_(J) (J=6−0) ground states were observed for Ga₄Tb(shi)₄ (FIG. 10), with a quantum yield of 34.7% in the solid state.

The desirable NIR emission from Ho³⁺ is extremely rare in systems containing organic lanthanide sensitizers. The NIR emission arising from Ga₄Ho(shi)₄ was observed at 965-990 nm and is due to the ⁵F₅→⁵I₇ transition and at 1160-1190 nm originating from the ⁵I₆→⁵I₈ transition. The quantum yield for Ga₄Ho(shi)₄ in the solid-state (2.0(2)·10⁻³%) is the first quantitative value reported. Emission signal in the visible was not observed, nor was it possible to collect an emission spectrum in protic solvents.

A Yb³⁺ emission signal was observed with an apparent maximum at 980 nm for Ga₄Yb(shi)₄ and is attributed to the ²F_(5/2)→²F_(7/2) transition. The relatively shorter distance between the Ln³⁺ and C—H oscillators in comparison to other, previously reported Zn₁₆Ln metallacrowns, is expected to result in additional deactivation by quenching through these oscillators. Moreover, in solution an additional source of quenching has been identified through the coordination of a solvent molecule due to the insufficient protection of Ln³⁺. Nevertheless, the quantum yield value that was measured for Ga₄Yb(shi)₄ in the solid state (5.88(2)%) is over 1.5 fold higher than any other comparable reported quantum yields for Yb³⁺ complexes with organic ligands containing C—H bonds. However, in solution, quantum yields are in the range of average values for other Yb³⁺ complexes and are significantly lower than those measured in the solid state. These results can be partially explained by the non-zero value of q where the deactivation from the solvent will take a higher importance. From these observations, it can be concluded that the intrinsic sensitization efficiency is relatively high for this system, as demonstrated by the high quantum yields in the solid state.

For the Er³⁺ complex Ga₄Er(shi)₄, the typical long wavelength emission is observed at ca. 1500-1600 nm originating from the ⁴I_(13/2)→⁴I_(15/2) transition. The quantum yield of this transition in the solid state is the same within experimental error as the value observed for the previously reported Zn₁₆Er metallacrown complexes, 4.4(1)·10⁻²% versus 4.2(1)·10⁻²%. This difference increases when the measurements are performed in deuterated solvent with the respective values of 4.5(3)·10⁻²% versus 3.60(6)·10⁻²%. It is believed that these values are among the highest ever reported.

Ga₈Ln₂(shi)₈ Complexes

Since the Ga₈Ln₂(shi)₈ dimer compounds were insoluble in all solvents, the solid state photophysical data alone were collected. As with the Ga₄Gd(shi)₄ compound, the Ga₈Gd₂(shi)₈ complex may be used to examine the ligand-based photophysical properties. At room temperature in the solid state, under 310 nm excitation, Ga₈Gd₂(shi)₈ exhibits broad-band ligand-centered luminescence in the range 350-650 nm with a maximum at 440 nm (22 727 cm⁻¹, FIG. 11). The solid state ligand phosphorescence was obtained at 77 K using an excitation wavelength of 350 nm and a 100 μs delay (FIG. 11). The profile of the phosphorescence is similar to that of Ga₄Gd(shi)₄ (FIG. 9), which was expected, since both compounds are formed by the same framework ligand, H₃shi. The 0-0 transition was determined to be located at 455 nm, or 21 978 cm⁻¹. This triplet state energy is slightly lower than that of the Ga₄Gd(shi)₄ compound (22 170 cm⁻¹), but still is sufficiently high in energy to sensitize visible (Dy³⁺, Tb³⁺, Sm³⁺) and/or NIR emitting (Dy³⁺, Sm³⁺, HO3+, Yb³⁺, Pr³⁺, Nd³⁺, Er³⁺) lanthanides.

The excitation spectra of Ga₈Ln₂(shi)₈ MCs exhibit profiles similar to the ones observed for the Ga₄Ln(shi)₄ complexes (FIG. 8): broad ligand-centered bands up to 350-400 nm and sharp characteristic f-f transitions in the case of Pr³⁺, Nd³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Ho³⁺ and Er³⁺ MCs (FIG. 12). It is noted that for Ga₈Nd₂(shi)₈ MC the intensity of f-f transitions is comparable with the ligand-centered transitions (FIG. 12).

Using an excitation wavelength of 350 nm, both visible and NIR emission arising from f-f transitions of different lanthanide ions can be observed (FIG. 13). Quantitative parameters are summarized in Table 4. In general, quantum yields are lower for Ga₈Ln₂(shi)₈ MCs compared to Ga₄Ln(shi)₄(shi)₈ (Table 3) except for Ho³⁺ analogues which quantum yield is increased by 1.65 times reaching 3.3(1)·10−3%.

TABLE 4 Photophysical data for Ga₈Ln₂(shi)₈ MC complexes. Q_(Ln) ^(L) (%) Q_(Ln) ^(L) (%) MC λ_(em)/nm τ_(obs)/μs ^(a) (visible) (NIR) Ga₈Pr₂(shi)₈ 1020  0.901(6) — 9.3(1) · 10⁻³ Ga₈Nd₂(shi)₈ 1064 2.46(1) — 0.99(2) Ga₈Sm₂(shi)₈ 597 117(1)    2.09(5)  0.269(3) Ga₈Tb₂(shi)₈ 545 1410(1)     31.2(2)  — Ga₈Dy₂(shi)₈ 575 15.0(1)  0.85(1) 7.5(1) · 10⁻² Ga₈Ho₂(shi)₈ 978  0.032(1) — 3.3(1) · 10⁻³ Ga₈Er₂(shi)₈ 1525 5.23(2) — 5.7(1) · 10⁻³ Ga₈Yb₂(shi)₈ 980 30.5(1)  — 2.43(6)

HeLa (Human Epithelial Ovarian Carcinoma) cell line obtained from ATCC (Molsheim, France) was cultured in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% heat-inactivated fetal bovine serum (FBS), 1% of 100× non-essential aminoacid solution, 1% of L-glutamine (GlutaMAX) and 1% of streptomycin/penycilin antibiotics. Cells were seeded in a 8-well Lab Tek Chamber coverglass (Nunc, Dutsher S.A., Brumath, France) at a density of 6×10⁴ cells/well and cultured at 37° C. in 5% humidified CO₂ atmosphere. After 24 hours, cell culture media was removed, and cells were washed twice with OPTIMEM (room temperature) and incubated with a solution of the Ga₈Yb₂(shi)₈ complex in OPTIMEM media supplemented with 2% of FBS at 37° C. in 5% CO₂ atmosphere during 3 hours. Prior to epifluorescent imaging the cells were washed twice with OPTIMEM (room temperature) in order to remove any non-specifically bound Ga₈Yb₂(shi)₈ complex. The cells were observed with a Zeiss Axio Observer Z1 fluorescence inverted microscope (Zeiss, Le Pecq, France) equipped with an EMCCD Evolve 512 (Roper Scientific) photometric camera. The light source, Zeiss HXP 120, was combined with the following filter cubes: 377 nm band pass 50 nm filter for the excitation and long pass filter 805 nm for Yb³⁺ emission in the NIR range. The

FIGS. 14A and B are microscope images of the Ga₈Yb₂(shi)₈ when incubated with the HeLa cells. FIG. 14A represents the brightfield (left), the NIR emission (center), and the merged (right) images, and FIG. 14B represents a control where no MC was added upon incubation.

The data presented in Example 1 presents a modular metallacrown platform that is highly efficient for the sensitization of visible and near-infrared lanthanide metal ions. Highly luminescent Ga₄Ln(shi)₄ coordination compounds can be obtained by the four-component self-assembly synthetic process disclosed herein. The size of the lanthanide has an impact on the formation of the complex; as the assembly for Ln³⁺ ions larger than Sm³⁺ could not be obtained. On the other hand, dimeric MCs Ga₈Ln₂ could be assembled for large lanthanides like Nd³⁺ and Pr³⁺. The electronic structure of the metallacrowns is remarkable in its ability to sensitize several lanthanide cations emitting in the visible (Eu³⁺, Tb³⁺, Tm³⁺) and in the near-infrared (Ho³⁺, Er³⁺, Yb³⁺, Pr³⁺, Nd³⁺) or in both (Sm³⁺ and Dy³⁺). The sensitization of a wide range of NIR emitting lanthanide cations opens new possibilities for multiplex bioanalytical experiments.

Unlike the LnZn₁₆L₁₆ sandwich complexes, the protection of the lanthanide cation against non-radiative deactivations in Ga₄Ln(shi)₄ MCs is not optimized but could be further improved. The design of these structures localizes the lanthanide cation at the center of the assembly, with the goal of precluding luminescence quenching arising from interactions with high-energy vibrations of surrounding molecules. However, a pocket between the bridging carboxylates provides a space for coordination of solvent molecules, as confirmed by the q values estimated from luminescence lifetimes in protic and deuterated solvents (Table 3). Nevertheless, the efficient sensitization ability of the H₃shi chromophores leads to highly luminescent visible and NIR Ln³⁺ emitters. It is believed that the results in this example are the highest quantum yield values for Yb³⁺ and Er³⁺ complexes containing organic ligands

Additionally, the Ga₈Ln₂(shi)₈ dimer compounds exhibited lower quantum yields and luminescent lifetimes than the Ga₄Ln(shi)₄ monomer complexes, reflecting the versatility of the disclosed systems and the possibility of easy tuning of photophysical properties.

The simplicity and synthetic reliability of the 12-MC-4 system provides ample opportunity to modulate structural features such as the framework and bridging ligands, in order to tune both the photophysical parameters and physical properties such as solution integrity, solubility, and add platforms for further coupling reactions. Such modifications include the extension of the aromatic ring of shi to naphthanoic hydroxamic acid, or the replacement of a CH unit with a nitrogen into the aromatic ring as shown in isonicotinic hydroxamic acid. Other modifications on shi and isophthalic acid include the addition of ethynyl, azido, isothiocyano, isocyano, and malimido substituents onto the aromatic rings.

As an example of utilization of copper-catalyzed alkyne azide cycloaddition, an ethynylsalicylhydroxamic acid derivative was coupled to an azido biotin derivative, both of which were prepared according to literature procedures. The reaction of these complexes in water:terf-butanol solution in the presence of cupric sulfate, sodium ascorbate (NaAsc, a reductant), and tri(benzyltriazylpropyl)amine (TBTA, a chelating agent) resulted in the formation of the coupled compound according to the following scheme:

ESI-MS and 1H-NMR of this compound are provided in FIGS. 31 and 32. This procedure should be adaptable for use directly on metallacrown complexes containing ethynyl or azido functionalization.

Example 2

Preparation of Complexes Ga₄Ln(shi)₆

H₃shi (153.1 mg, 1.0 mmol), Ln(NO₃)₃.xH₂O (0.25 mmol), and Ga(NO₃)₃.xH₂O (255.7 mg, 1.0 mmol) were dissolved in 10 mL methanol. Pyridine (2 mL) and acetic acid (0.1 mL) were added and the mixture was stirred for 2 hours. The solution was filtered and kept undisturbed. X-ray quality crystals form after one day.

[GdGa₄(shi)₄(H₂shi)₂(C₅H₅N)₄(NO₃)].(C₅H₅N)₂ (Ga₄Gd(shi)₆). Yield: 131 mg (28%). ESI-MS, calc. for [M]⁻, C₄₂H₂₆N₆O₁₈GdGa₄, 1338.8; found, 1337.9. Anal. Calcd for GdGa₄C₇₂H₅₈N₁₃O₂₁: C, 46.06; H, 3.11; N, 9.70. Found: C, 46.02; H, 3.07; N, 9.33.

[TbGa₄(shi)₄(H₂shi)₂(C₅H₅N)₄(NO₃)].(C₅H₅N)₂ (Ga₄Tb(shi)₆). Yield: 153 mg (33%). ESI-MS, calc. for [M]⁻, C₄₂H₂₆N₆O₁₈GdGa₄, 1340.5; found, 1339.8. Anal. Calcd for TbGa₄C₇₂H₅₈N₁₃O₂₁: C, 46.02; H, 3.11; N, 9.69. Found: C, 46.31; H, 3.20; N, 9.64.

[DyGa₄(shi)₄(H₂shi)₂(C₅H₅N)₄(NO₃)].(C₅H₅N)₂ (Ga₄Dy(shi)₆). Yield: 151 mg (32%). ESI-MS, calc. for [M]⁻, C₄₂H₂₆N₆O₁₈DyGa₄, 1344.1; found, 1343.9. Anal. Calcd for DyGa₄C₇₂H₅₈N₁₃O₂₁: C, 45.93; H, 3.11; N, 9.67. Found: C, 46.22; H, 3.17; N, 9.49.

[HoGa₄(shi)₄(H₂shi)₂(C₅H₅N)₄(NO₃)].(C₅H₅N)₂ (Ga₄Ho(shi)₆). Yield: 147 mg (31%). ESI-MS, calc. for [M]⁻, C₄₂H₂₆N₆O₁₈HoGa₄, 1346.5; found, 1346.8. Anal. Calcd for HoGa₄C₇₂H₅₈N₁₃O₂₁: C, 45.87; H, 3.10; N, 9.66. Found: C, 46.10; H, 3.13; N, 9.48.

[ErGa₄(shi)₄(H₂shi)₂(C₅H₅N)₅].(NO₃).(C₅H₅N) (Ga₄Er(shi)₆). Yield: 127 mg (27%). ESI-MS, calc. for [M]⁻, C₄₂H₂₆N₆O₁₈ErGa₄, 1348.9; found, 1347.9. Anal. Calcd for ErGa₄C₇₂H₅₈N₁₃O₂₁: C, 45.82; H, 3.10; N, 9.65. Found: C, 45.68; H, 3.37; N, 9.35

[TmGa₄(shi)₄(H₂shi)₂(C₅H₅N)₅].(NO₃).(C₅H₅N) (Ga₄Tm(shi)₆). Yield: 166 mg (35%). ESI-MS, calc. for [M]⁻, C₄₂H₂₆N₆O₁₈ErGa₄, 1350.5; found, 1350.8. Anal. Calcd for TmGa₄C₇₂H₅₈N₁₃O₂₁: C, 45.78; H, 3.09; N, 9.64. Found: C, 45.48; H, 3.35; N, 9.26.

[YbGa₄(shi)₄(H₂shi)₂(C₅H₅N)₅].(NO₃).(C₅H₅N)₂ (Ga₄Yb(shi)₆). Yield: 114 mg (24%). ESI-MS, calc. for [M]⁻, C₄₂H₂₆N₆O₁₈ErGa₄, 1354.6; found, 1353.8. Anal. Calcd for YbGa₄C₇₂H₅₈N₁₃O₂₁: C, 45.68; H, 3.09; N, 9.62. Found: C, 45.85; H, 3.30; N, 9.31.

The X-ray crystal structure of the representative molecule Ga₄Tb(shi)₆ is shown in FIG. 2A. The complex crystallizes in the triclinic space group Pī. Compounds Ga₄Ln(shi)₆ (Ln=Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺) contain a non-planar Ln³⁺(12-MC_(Ga) ^(III) _(N(Shi))-4) core (shown in FIG. 2B for Tb-2). The central Ln ion is bridged to two gallium ions by two H₂shi⁻ ligands and its coordination sphere is filled by a chelating nitrate.

The UV-Vis spectroscopy data for the H₃shi and the Ga₄Ln(shi)₆ molecules in methanol solution are shown in FIG. 15, the diffuse reflectance spectra of the Ga₄Ln(shi)₆ molecules are shown in FIG. 16A, the excitation and emission spectra of Ga₄Ln(shi)₆ complexes are respectively shown in FIGS. 17 and 18.

Table 5 depicts the photophysical parameters of some of the solid state Ga₄Ln(shi)₆ molecules.

TABLE 5 Photophysical parameters of solid state Ga₄Ln(shi)₆ Q_(Ln) ^(L)/% Compound VIS NIR τ/μs Ga₄Tbshi₆ 11.3(5) 509(7)    Ga₄Dyshi₆ 0.308(8) (total) n.a. 3.36(6) 0.222(6) (only Dy) Ga₄Hoshi₆ n.a.  0.032(1) Ga₄Ershi₆ 1.55(3) · 10⁻³  0.220(3) Ga₄Ybshi₆ 0.216(6) 2.99(2)

Example 3

Synthetic procedure for [LnGa₆(shi)₇(Hshi)(H₂shi)(C₆H₁₆N)₃(C₅H₅N)₂]xH₂O [Ga₆Ln(shi)₉]

Ga(NO₃)₃.xH₂O (273.8 mg, 1.0 mmol), and Ln(NO₃)₃.xH₂O (0.1667 mmol) were dissolved in 10 mL of methanol. Separately, H₃shi (229.8 mg, 1.5 mmol), triethylamine (0.62 mL, 4.5 mmol) and pyridine (10 mL) were mixed, then added to the metal salt solution, and was left to stir overnight. The solution was filtered and allowed to crystallize via diffusion of diethyl ether. Crystals were observed in 1-2 weeks.

[PrGa₆(shi)₇(Hshi)(H₂shi)(C₆H₁₆N)₃(C₅H₅N)₂].4H₂O [Ga₆Pr(shi)₉]: Yield: 116.5 mg (28%). Elemental Analysis for PrGa₆C₉₁H₁₀₅N₁₄O₃₁ (Calcd): 44.98% C, (44.61); 4.28% H, (4.32); 8.22% N, (8.00).

[NdGa₆(shi)₇(Hshi)(H₂shi)(C₆H₁₆N)₃(C₅H₅N)₂].7H₂O [Ga₆Nd(shi)₉]: Yield: 68.1 mg (2%). ESI-MS for [M]⁻ NdGa₆C₆₃H₄₁N₉O₂₇ (Calcd): 1917.2 (1914.7). Elemental Analysis for NdGa₆C₉₁H₁₁₁N₁₄O₃₄ (Calcd): 43.59% C, (43.59); 4.30% H, (4.46); 7.87% N, (7.82).

[SmGa₆(shi)₇(Hshi)(H₂shi)(C₆H₁₆N)₃(C₅H₅N)₂].6H₂O [Ga₆Sm(shi)₉]: Yield: 134.2 mg (33%). ESI-MS for [M]⁻ SmGa₆C₆₃H₄₁N₉O₂₇ (Calcd): 1925.3 (1924.76). Elemental Analysis for SmGa₆C₉₁H₁₀₉N₁₄O₃₃ (Calcd): 43.81% C, (43.80); 4.32% H, (4.40); 8.00% N, (7.86).

[TbGa₆(shi)₇(Hshi)(H₂shi)( C₆H₁₆N)₃(C₅H₅N)₂] [Ga₆Tb(shi)₉]: Yield: 138.5 mg (36%). ESI-MS for [M]⁻ TbGa₆C₆₃H₄₁N₉O₂₇ (Calcd): 1932.1 (1933.32). Elemental Analysis for TbGa₆C₉₁H97N₁₄O₂₇ (Calcd): 45.43% C, (45.62); 4.23% H, (4.08); 8.19% N, (8.18).

[DyGa₆(shi)₇(Hshi)(H₂shi)(C₆H₁₆N)₃(C₅H₅N)].6H₂O [Ga₆Dy(shi)₉]: Yield: 98.8 mg (24%). ESI-MS for [M]⁻ DyGa₆C₆₃H₄₁N₉O₂₇ (Calcd): 1936.9 (1936.9). Elemental Analysis for DyGa₆C₉₁H₁₁₁N₁₄O₃₄ (Calcd): 42.48% C, (42.53); 4.21% H, (4.32); 7.60% N, (7.50).

[HoGa₆(shi)₇(Hshi)(H₂shi)(C₆H₁₆N)₃(C₅H₅N)₂].8H₂O [Ga₆Ho(shi)₉]: Yield: 49.4 mg (12%). Elemental Analysis for HoGa₆C₉₁H₁₁₃N₁₄O₃₅ (Calcd): 42.63% C, (42.93); 4.38% H, (4.47); 7.64% N, (7.70)

[ErGa₆(shi)₇(Hshi)(H₂shi)(C₆H₁₆N)₃(C₅H₅N)₂].6H₂O [Ga₆Er(shi)₉]: Yield: 40.8 mg (10%). ESI-MS for [M]⁻ SmGa₆C₆₃H₄₁N₉O₂₇ (Calcd): 1941.0 (1939.7). Elemental Analysis for ErGa₆C₉₁H₁₀₉N₁₄O₃₃ (Calcd): 43.39% C, (43.50); 4.36% H, (4.37); 7.78% N, (7.80)

[YbGa₆(shi)₇(Hshi)(H₂shi)(C₆H₁₆N)₃(C₅H₅N)₂].8H₂O [Ga₆Yb(shi)₉]: Yield: 102.5 mg (24%). ESI-MS for [M]⁻ YbGa₆C₆₃H₄₁N₉O₂₇ (Calcd): 1947.0 (1946.7). Elemental Analysis for YbGa₆C₉₁H₁₁₃N₁₄O₃₅ (Calcd): 42.85% C, (42.79); 4.18% H, (4.46); 7.71% N, (7.68)

FIGS. 19A-C show a crystal structure of Ga₆Tb(shi)₉, which crystallizes in Pī. The central Tb³⁺ ion is nine coordinate, binding to the oximate oxygens of nine H₃shi ligands, with a geometry described as tricapped trigonal prism. Four of the gallium ions (Ga2, Ga4, Ga5, and Ga6) are in distorted octahedral environments with propeller confirmations; Ga2 and Ga4 are ∧ while Ga5 and Ga6 are Δ chirality. The remaining gallium ions (Ga1 and Ga3) are five coordinate, with a geometry closer to square pyramidal, τ=0.2525 and 0.2697 respectively.

The UV-Vis spectroscopy data for the H₃shi and the Ga₆Ln(shi)₉ MCs in methanol solution are shown in FIG. 20, and the luminescence data for Ga₆Ln(shi)₉ are shown in FIGS. 21A and 21B.

Table 6 depicts the photophysical parameters of some of the solid state Ga₆Ln(shi)₉ complexes.

TABLE 6 Photophysical data of Ga₆Ln(shi)₉ complexes in the solid state Compound Q_(Ln) ^(L)/% τ/μs Ga₆Pr(shi)₉ 3.7(2) · 10⁻³ 0.063(1) Ga₆Nd(shi)₉ 0.171(5) 0.71(1)  Ga₆Sm(shi)₉ 1.64(9)  70(1)    Ga₆Tb(shi)₉ 0.189(3) 20.7(5): 71% 4.54(6): 29% Ga₆Ho(shi)₉ 1.1(2) · 10⁻³ 0.037(1) Ga₆Er(shi)₉ 7.1(2) · 10⁻³ 0.905(8) Ga₆Yb(shi)₉ 0.65(3)  7.26(2) 

Preparation of [Ln₂Ga₈(shi)₈(mip)₄](CH₃OH)₈(C₅H₆N⁺)₂ Complexes

Preparation of 5-maleimidoisophthalic Acid (H₂mip)

5-aminoisophthalic acid hydrate (19.92 g, 0.10 mol) and maleic anhydride (9.81 g, 0.10 mol) were dissolved in 200 mL of DMF. The mixture was stirred for ˜1 hour after which DMF was removed under vacuum. The obtained product was washed with acetone to give ˜19.00 g of 5-[(3-carboxy-1-oxo-2-propen-1-yl)amino]isophthalic acid as a yellow powder. 2.79 g (0.01 mol) of 5-[(3-carboxy-1-oxo-2-propen-1-yl)amino]isophthalic acid was added in to the solution of acetic anhydride (15.0 mL) and sodium acetate trihydrate (0.68 g, 0.005 mol) and the mixture was stirred at 60° C. for 2.5 hours. Acetic anhydride was then removed under vacuum and water (20.00 mL) was added. The slurry mixture was stirred at 70° C. for another 2 hours, filtered, and washed with copious amount of water. The white solid obtained is dried under vacuum to give pure 5-maleimidoisophthalic acid. Yield: ˜2.0 g. Elemental analysis (%) for C₁₂H₇NO₆ (Calcd): C, 55. 10 (55.18); H, 2.71 (2.70); N, 5.48 (5.36). ¹H-NMR (400 MHz, DMSO) δ (ppm) 8.49 (1H, s), 8.18 (2H, s), 7.24 (2H, s). ¹³C-NMR (400 MHz, DMSO) δ (ppm) 169.66, 166.18, 134.85, 132.60, 132.30, 130.81, 128.69.

Preparation of [Ln₂Ga₈(shi)₈(mip)₄](CH₃OH)₈(C₅H₆N⁺)₂

5-maleimidoisophthalic acid (0.26 g, 1.00 mmol) and [LnGa₄(shi)₄C₆H₅CO₂)₄(C₄H₄N)(CH₃OH)] (0.20 mmol) were dissolved in DMF (20 mL). The mixture was stirred for 4 hours and DMF was evaporated under vacuum. The obtained powder was washed with copious amount of MeOH to give the pure product.

[Tb₂Ga₈(shi)₈(mip)₄](CH₃OH)₈(C₅H₆N⁺)₂ (Tb₂Ga₈(mip)₄). Yield: 0.28 g (79%). ESI-MS, calc. for [M]²⁻, C₁₀₄H₅₂Ga₈N₁₂O₄₈Tb₂, 1563.2; found, 1563.2. Anal. Calcd for C₁₂₂H₉₆Ga₈N₁₄O₅₆Yb₂: C, 41.51; H, 2.74; N, 5.56; found: C, 41.65; H, 2.61; N, 5.35.

[Yb₂Ga₈(shi)₈(mip)₄](CH₃OH)₈(C₅H₆N⁺)₂ (Yb₂Ga₈(mip)₄). Yield: 0.30 g (84%). ESI-MS, calc. for [M]²⁻, C₁₀₄H₅₂Ga₈N₁₂O₄₈Yb₂, 1570.7; found, 1570.7 (FIG. 27). Anal. Calcd for C₁₂₂H₉₆Ga₈N₁₄O₅₆Yb₂: C, 41.18; H, 2.72; N, 5.51; found: C, 41.61; H, 2.60; N, 5.34.

Preparation of [Ln₂Ga₈(shi)₈(itip)₄](CH₃OH)₈(C₅H₆N⁺)₂ Complexes

Preparation of 5-isothiocyanateisophthalic Acid (H₂itip)

5-aminoisophthalic acid hydrate (1.17 g, 5.90 mmol) was dissolved in 12 mL acetone in which 25 mL solution of sodium acetate (1 M) in water was added. The mixture was cooled down to 0° C. and sodium hydroxide (0.50 g, 12.5 mmol) was added to yield a homogeneous solution. Thiophosgene (0.5 mL) was then added and the mixture was stirred for 30 minutes, after which the pH of the solution was adjusted to ˜3 by hydrochloric acid. The precipitate was filtered and dissolved in acetone. The obtained solution was filtered and the filtrate was concentrated under vacuum to yield a white powder. Recrystallization in MeOH gave the pure product. Yield: 0.70 g. Elemental analysis (%) for C₉H₅NO₄S (Calcd): C, 48.51 (48.43); H, 2.32 (2.22); N, 6.40 (6.28).

¹H-NMR (400 MHz, DMSO) δ (ppm) 13.64 (2H, s), 8.37 (1H, s), 8.08 (2H, s). ¹³C-NMR (400 MHz, DMSO) δ (ppm) 165.61, 136.39, 133.27, 131.62, 130.44, 128.81.

Preparation of [Ln₂Ga₈(shi)₈(itip)₄](CH₃OH)₈(C₅H₆N⁺)₂

5-isothiocyanateisophthalic acid (0.036 g, 1.60 mmol) and

[LnGa₄(shi)₄(C₆H₅CO₂)₄(C₅H₅N)(CH₃OH)] (0.04 mmol) were dissolved in DMF (4 mL). The mixture was stirred for 4 hours and DMF was evaporated under vacuum. The obtained powder was washed with copious amount of MeOH to give the pure product.

[Tb₂Ga₈(shi)₈(itip)₄](CH₃OH)₈(C₅H₆N⁺)₂ (Tb₂Ga₈(itip)₄). Yield: 0.019 g (28%). ESI- MS, calc. for [M]²⁻, C₉₂H₄₄Ga8N₁₂O₄₀S₄Yb₂, 1479.7; found, 1479.7. Anal. Calcd for C₁₁₀H₈₈Ga₈N₁₄O₄₈S₄Tb₂: C, 39.11; H, 2.63; N, 5.81; found: C, 39.25; H, 2.61; N, 5.95.

[Yb₂Ga₈(shi)₈(itip)₄](CH₃OH)₈(C₅H₆N⁺)₂ (Yb₂Ga₈(itip)₄). Yield: 0.021 g (31%). ESI- MS, calc. for [M]²⁻, C₉₂H₄₄Ga₈N₁₂O₄₀S₄Yb₂, 1494.7; found, 1494.7 (FIG. 28). Anal. Calcd for C₁₁₀H₈₈Ga₈N₁₄O₄₈S₄Yb₂: C, 38.79; H, 2.60; N, 5.76; found: C, 39.01; H, 2.66; N, 6.09.

Preparation of [Ln₂Ga₈(shi)₈(thiol-mip)₄](CH₃OH)₈(C₅H₆N⁺)₂ Complexes

[Ln₂Ga₈(shi)₈(mip)₄](CH₃OH)₈(C₅H₆N⁺)₂ (0.020 mmol) and a thiol-bearing compound (0.08 mmol) were mixed in 2 mL DMF. After 4 hours, DMF was evaporated under vacuum and the obtained powder was washed with MeOH to give the pure product.

[Yb₂Ga₈(shi)₈(L-cysteine-mip)₄](CH₃OH)₈(C₅H₆N⁺)₂ (Yb₂Ga₈(L-cysteine-mip)₄). Yield: 0.060 g (74%). ESI-MS, calc. for [M]²⁻, C₁₁₆H₈₀Ga₈N₁₆O₅₆S₄Yb₂, 1812.7; found, 1811.8 (FIG. 29). Anal. Calcd for C₁₃₄H₁₂₄Ga₈N₁₈O₆₄S₄Yb₂: C, 39.81; H, 3.09; N, 6.24; found: C, 39.69; H, 3.10; N, 5.94.

[Yb₂Ga₈(shi)₈(cysteamine-mip)₄](CH₃OH)₈(C₅H₆N⁺)₂ (Yb₂Ga₈(cysteamine-mip)₄). Yield: 0.065 g (83%). ESI-MS, calc. for [M]²⁻, C112H₈₀Ga₈N₁₆O₄₈S₄Yb₂, 1724.8; found, 1724.9 (FIG. 30). Anal. Calcd for C₁₃₀H₁₂₈Ga₈N₁₈O₅₈S₄Yb₂: C, 40.01; H, 3.31; N, 6.46; found: C, 40.36; H, 3.20; N, 6.74.

Preparation of [Na₂Ln₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄ Complexes

H₃shi (306.3 mg, 2.0 mmol), Ln(NO₃)₃.xH₂O (0.50 mmol), Ga(NO₃)₃.xH₂O (511.5 mg, 2.0 mmol), and 5-sulfoisophthalic acid sodium salt (268.2 mg, 1.0 mmol) were dissolved in 18 mL DMF. Ammonium bicarbonate (632.5 mg, 8.0 mmol) was added to the solution and stirred overnight. The solution was filtered. Slow evaporation of the solution yielded crystalline compound after 1 month.

[Na₂Nd₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄ (Ga₈Nd₂(sip)₄). Yield: 351 mg (32%). Anal. Calcd for C₁₃₀H₁₇₈Ga₈N₂₆Na₂Nd₂O₇₆S₄: C, 35.97; H, 4.13; N, 8.39; found: C, 36.45; H, 4.45; N, 8.78.

[Na₂Sm₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄(Ga₈Sm₂(sip)₄). Yield: 360 mg (33%). Anal. Calcd for C₁₃₀H₁₇₈Ga8N₂₆Na₂Sm₂O₇₆S₄: C, 35.86; H, 4.12; N, 8.36; found: C, 36.37; H, 4.43; N, 8.78.

[Na₂Eu₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄(Ga₈Eu₂(sip)₄). Yield: 320 mg (29%). Anal. Calcd for C₁₃₀H₁₇₈Ga₈N₂₆Na₂Eu₂O₇₆S₄: C, 35.84; H, 4.12; N, 8.36; found: C, 36.20; H, 4.38; N, 8.72.

[Na₂Gd₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄(Ga₈Gd₂(sip)₄). Yield: 375 mg (34%). Anal. Calcd for C₁₃₀H₁₇₈Ga₈N₂₆Na₂Gd₂O₇₆S₄: C, 35.75; H, 4.11; N, 8.34; found: C, 36.18; H, 4.35; N, 8.74.

[Na₂Tb₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄(Ga₈Tb₂(sip)₄). Yield: 367 mg (34%). Anal. Calcd for C₁₃₀H₁₇₈Ga₈N₂₆Na₂Tb₂O₇₆S₄: C, 35.72; H, 4.11; N, 8.33; found: C, 36.10; H, 4.35; N, 8.72.

[Na₂Dy₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄(Ga₈Dy₂(sip)₄). Yield: 344 mg (31%). Anal. Calcd for C₁₃₀H₁₇₈Ga₈N₂₆Na₂Dy₂O₇₆S₄: C, 35.67; H, 4.10; N, 8.32; found: C, 36.07; H, 4.39; N, 8.72.

[Na₂Ho₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄(Ga₈Ho₂(sip)₄). Yield: 343 mg (31%). Anal. Calcd for C₁₃₀H₁₇₈Ga₈N₂₆Na₂Ho₂O₇₆S₄: C, 35.63; H, 4.09; N, 8.31; found: C, 35.42; H, 4.32; N, 8.55.

[Na₂Er₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄(Ga₈Er₂(sip)₄). Yield: 311 mg (28%). Anal. Calcd for C₁₃₀H₁₇₈Ga₈N₂₆Na₂Er₂O₇₆S₄: C, 35.59; H, 4.09; N, 8.30; found: C, 35.72; H, 4.31; N, 8.50.

[Na₂Tm₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄(Ga₈Tm₂(sip)₄). Yield: 332 mg (30%). Anal. Calcd for C₁₃₀H₁₇₈Ga₈N₂₆Na₂Tm₂O₇₆S₄: C, 35.56; H, 4.09; N, 8.29; found: C, 35.56; H, 4.44; N, 8.51.

[Na₂Yb₂Ga₈(shi)₈(sip)₄(H₂O)₁₀](C₃H₇NO)₁₄(NH₄)₄(Ga₈Yb₂(sip)₄). Yield: 337 mg (31%). Anal. Calcd for C₁₃₀H₁₇₈Ga₈N₂₆Na₂Yb₂O₇₆S₄: C, 35.49; H, 4.08; N, 8.28; found: C, 35.52; H, 4.40; N, 8.56.

As illustrated throughout the examples, upon excitation through their organic chromophoric moieties, the heterometallic metallacrowns disclosed herein exhibited remarkable luminescence properties across the visible and near-infrared (NIR) regions. In some instances, the remarkable luminescence properties were in spite of the presence of solvent molecules bound in the first sphere of coordination of Ln³⁺ ions. It is believed that the sensitization efficiency in this system is sufficiently high, and thus outweighs the contribution of the quenching process. Several complexes of this series (e.g., Ga₄Yb(shi)₄, and Ga₄Er(shi)₄) display the highest reported quantum efficiencies values in the solid state (in comparison to other lanthanide complexes formed with organic ligands currently described in the literature). In solution, interactions with the solvent take an increased importance, lowering the luminescence intensity through non-radiative quenching processes. The examples disclosed herein also demonstrate NIR luminescence for a Dy³⁺ (Ga₄Dy(shi)₄,) complex formed with the metallacrown in solution, as well as the less common visible and NIR luminescence, respectively, for Tm³⁺ and Ho³⁺ (Ga₄Tm(shi)₄, and Ga₄Ho(shi)₄,) in metallacrown complexes in the solid state.

The luminescent lanthanide complexes disclosed herein hold great promise for bioanalytical assays and biological imaging as their optical properties have several advantages over classical organic fluorophores and semi-conductor nanoparticles. Attractive luminescence characteristics include long luminescence lifetimes, large energy differences between excitation and emission bands and sharp emission bands throughout the visible and near-infrared (NIR) spectral ranges. These emission bands do not overlap and their wavelength positions are not affected by variation of the local microenvironment, such as changes in polarity, temperature, pH, or interactions with biological media. It is believed that the luminescent lanthanide complexes disclosed herein also do not photobleach.

The complexes disclosed herein, with such strong NIR luminescence, may be suitable for use in a broad range of applications including bioanalytical assays, biological imaging, telecommunications, energy conversion, barcodes, and optical materials. Additionally, these complexes have potential for multi-modal applications. Coupled with the strong luminescence, the presence of Ga³⁺ ions allows for use of radioactive gallium for positron emission tomography (Ga-68) or single-photon emission computed tomography (Ga-67). The lanthanide ion in this system also has access to approximately one solvent molecule (q=1) which opens the opportunity for imaging via magnetic resonance techniques (MRI, CEST, etc).

It is to be understood that any ranges provided herein include the stated range and any value or sub-range within the stated range. Furthermore, when “about” is utilized to describe a value, this is meant to encompass minor variations (up to +/−10%) from the stated value.

Reference throughout the specification to “one example”, “another example”, “an example”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the example is included in at least one example described herein, and may or may not be present in other examples. In addition, it is to be understood that the described elements for any example may be combined in any suitable manner in the various examples unless the context clearly dictates otherwise.

In describing and claiming the examples disclosed herein, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.

While several examples have been described in detail, it will be apparent to those skilled in the art that the disclosed examples may be modified. Therefore, the foregoing description is to be considered non-limiting. 

1. A heterometallic metallacrown compound incorporating Ga(III) and Ln(III) cations, with a templating ligand, wherein said metallacrown contains at least one repeating [—Ga—N—O—] sub-unit where the N—O derives from the templating ligand.
 2. The heterometallic metallacrown compound of claim 1, including a Ln(III)[12-MC_(Ga) ^(III) _(N(shi))-4] core, wherein MC_(Ga) ^(III) _(N(shi)) is a metallacrown macrocycle with a repeating sub-unit consisting of Ga(III) ion and a salicylic hydroxamic acid (H₃shi) ligand or its derivatives.
 3. The heterometallic metallacrown compound of any one of claims 1 to 2, including at least one counteraction (C⁺) which balances the charge of the compound.
 4. The heterometallic metallacrown compound of claim 1, including at least one ligand as bridging unit between the Ga(III) ion and the Ln(III) ion.
 5. The heterometallic metallacrown compound of claim 4, wherein the ligand contains at least one carboxylate group COO⁻.
 6. The heterometallic metallacrown compound of claim 4, wherein the ligand is an isophthalate group or derivatives thereof.
 7. The heterometallic metallacrown compound of claim 1, having the formula: Ln(III)(OX)₄[12-MC_(Ga) ^(III) _(N(shi))-4](C⁺), wherein MC_(Ga) ^(III) _(N(shi)) is a metallacrown macrocycle with a repeating sub-unit consisting of Ga(III) ion and a salicylic hydroxamic acid (H₃shi) ligand or its derivatives, OX⁻ are bridging carboxylate units and C+ are countercations that balance the charge of the compound.
 8. The heterometallic metallacrown compound of claim 1, having the formula: Ln₂(isophthalate)₄[12-MC_(GaNshi)-4]₂.
 9. The heterometallic metallacrown compound of claim 1, having the formula: Ln³⁺[12-MC_(Ga) ^(III) _(N(shi))-4].
 10. The heterometallic metallacrown compound of claim 1, having the formula: Ga₆Ln(shi)₉.
 11. The heterometallic metallacrown compound of claim 1, wherein Ln(III) is chosen from the lanthanide ions consisting of: yttrium (Y³⁺), lanthanum (La³⁺), cerium (Ce³⁺), praseodymium (Pr³⁺), neodymium (Nd³⁺), promethium (Pm³⁺), samarium (Sm³⁺), europium (Eu³⁺), gadolinium (Gd³⁺), terbium (Tb³⁺), dysprosium (Dy³⁺), holmium (Ho³⁺), erbium (Er³⁺), thulium (Tm³⁺), ytterbium (Yb³⁺), and lutetium (Lu³⁺).
 12. A method for bioanalytlcal assay or biological imaging, comprising the use of the heterometallic metallacrown compound of claim
 1. 13. The heterometallic metallacrown compound of claim 4, wherein the ligand is a benzoate group. 